1. Field of the Invention
This invention relates generally to optical communication systems and, more particularly, to optical communication systems which can adapt to the fiber-optic network topology to more effectively transmit data over the communication infrastructure.
2. Description of the Related Art
There is an ever increasing demand for increased capacity of network communication systems, which has led to a corresponding increase in data rates, as well as higher channel densities. Along with such higher data rates and higher channel densities come degrading effects, including signal spectral truncation due to optical filtering, linear WDM crosstalk, and nonlinear crosstalk due to inter-channel fiber nonlinear effects. Thus, there is an interest to provide more efficient modalities of transmitting data over a fiber-optic network infrastructure, as part of a network communication system. Such modalities include converting data into a proper modulation format, or data waveform format, prior to transmission. In particular, the signal pulse shape is an important design choice for optimizing transmission performance. Typical pulse shapes utilized today in the modulation process include non-return-to-zero (NRZ), and return-to-zero (RZ) with various duty cycles (e.g. pulses with 33%, 50% and 67% duty cycle can be most easily implemented using a pulse-carver Mach-Zehnder modulator (MZM)), as well as other existing formats, many of which are derived from, or further manipulation of, the NRZ or RZ formats. With reference to FIG. 5, a graph 700 depicts exemplary basic RZ and NRZ pulse shapes, each of which depicting the data bit pattern in encoded form relative to time. Moreover, in polarization multiplexed transmission, for example polarization multiplexed DQPSK (PM-DQPSK), e.g. RZ PM-DQPSK or NRZ PM-DQPSK, an addition degree of freedom exists. More specifically, the pulses of each of the two orthogonal polarization channels within a polarization multiplexed transmission can overlap in time, referred herein as “aligned” format, or can be offset by half a symbol period, referred herein to “interleaved” format. Each such format, however, has certain advantages and disadvantages relative to the network topology of the fiber-optic communication network. For example, interleaved formats typically have the best tolerance to nonlinear polarization scattering impairments; however, interleaved formats suffer greater penalty for polarization mode dispersion compared with aligned formats. RZ formats typically have a better tolerance to nonlinear transmission effects compared with NRZ formats but may suffer more spectral truncation by optical filters due to a wider spectrum of RZ signals. Therefore, network communication systems which rely on a single modulation format are less flexible to adapt to changing or variable network topologies, resulting in increased costs or inefficient operation of the communication system.
What is needed is an optical transmitter that can dynamically switch from one modulation format to another modulation format in response to real-time changes in the network topology of a network communication system, such that the optical transmitter continuously optimizes itself to the changes in the network topology. Further, what is needed is a transmitter which can provide such switching based upon certain characteristics of the network topology, some of which may change due to intentional switching of network optical paths, or with changing environmental conditions such as temperature. Still, what is needed is an optical transmitter which provides such switching in the electrical domain, prior to optical modulation. Still further, what is needed is a transmitter which can modify the data waveform prior to optical modulation to further reduce undesirable effects due to changes in the network topology.